Various embodiments of methods and apparatus for producing cement clinker are known in the art. Fresh raw material or a fresh raw material mixture of powdered and fine-grained raw material components conventionally is preheated in a multi-stage suspension preheater, generally in a cyclone preheater. In this case the procedure is generally such that either the entire raw material passes through the preheater from top to bottom in general counterflow to hot kiln exhaust gases and is thereby preheated, whereupon this preheated material is then largely deacidified in a precalcination arrangement, or a proportion of the raw material which has been partially or thoroughly preheated is branched off in a lower cyclone stage, calcined in a separate precalcination arrangement with fuel and air for combustion delivered and is then introduced with the rest of the preheated raw material into the inlet of a kiln (generally a rotary kiln) in the first section of which the preheated and precalcined material is first of all subjected to final calcining and then burnt to clinker. The hot clinker leaving the kiln is then cooled in a cooler by means of cooling air. The exhaust gases leaving the preheater from its uppermost stage can then after suitable cooling be freed of entrained fine dust in a filter arrangement, and this cooling of the exhaust gases can generally take place in a special cooling tower and/or in a raw material grinding installation (raw mill).
In many known constructions, and particularly in cases where a relatively large proportion of powdered raw material is fed to the preheater, problems always occur because a relatively large proportion of fine dust is precipitated in the filter arrangement during extraction of dust from the exhaust gases from the preheater. Efforts have indeed been made to return this fine dust to the raw material mixture, and this can be done for example in the raw material grinding installation or by delivering this fine dust from the filter directly to a preheater stage. Practice has shown, however, that this fine dust and the rest of the cement raw material can only be mixed with relative difficulty, so that a type of dust circulation frequently builds up in the region at least between the uppermost preheater stage and the filter arrangement, which is undesirable because on the one hand the filter arrangement is too severely loaded and on the other hand adulteration of the material composition occurs in the finished cement clinker.
The object of the invention, therefore, is to provide an improved method and apparatus in which fine dust filtered out of the exhaust gases from the preheater can likewise be very reliably preheated, precalcined and introduced into the kiln whilst at the same time the filter arrangement can be relieved at least largely of fine dust circulation.